a) Field of the Invention
The present invention relates to a photocatalytic substance which operates in visible light, and more particularly to an improvement of the catalytic function of such a photocatlytic substance.
b) Description of the Related Art
The use of titanium oxide as a photocatalytic substance is known conventionally, and its use in various fields is being studied. In known photocatalysts using titanium oxide, catalytic function is observed under exposure to ultraviolet light with a wavelength shorter than 410 nm, but is not observed under visible light with a longer wavelength. Only about 5% of sunlight, falls in the ultraviolet light range, so that known titanium oxide-based photocatalysts do not exhibit a sufficient catalytic function under sunlight. Thus, an ultraviolet light source, such as a mercury lamp, is sometimes separately provided to irradiate ultraviolet light on the photocatalyst in order to exhibit sufficient photocatalytic function.
Photocatalysts which can use visible light as operation light are also being studied, and there have been reports concerning such photocatalysts.
For example, there is proposed a method of adsorbing to titanium oxide a pigment which absorbs visible light. Some methods of doping a metallic element such as Cr or V to titanium oxide have been reported in the past 30 years. In addition, Japanese Patent Application Laid-Open Publication No. Hei 9-262482 discloses a method of doping a metallic element such as Cr or V to titanium oxide by an ion implantation technology and then applying a heat treatment.
However, photocatalysts having an adsorbed pigment are disadvantageous in that the pigment has a short life, and that they do not constantly exhibit sufficient catalytic function. According to the metal doping method, the catalytic function will probably not improve because the doped metal elements form an oxide and aggregate with metals. Therefore, reproducibility in manufacturing is poor, and, even when doping is successful, there is a problem that functionality tends to deteriorate as the device is used. As compared with simple metallic doping, functionality can be made more stable by employing ion implantation technology. However, ion implantation methods are disadvantageous in that the production costs are high, which makes difficulty in practical use except particular application.
The present invention was achieved in view of the above circumstances and it is an object of the present invention to provide a photocatalytic substance which can exert a stable photocatalytic function when irradiated by light in a visible range, and which can be produced easily and at a low cost.
The photocatalytic substance according to the present invention comprises a photocatalyst material, MOaXb (M is metal, O is oxygen, X is any element, a=1.5 to 2.0, b=0.01 to 0.5), which operates under visible light, or is activated by visible light, supported on a porous medium. Supporting the photocatalyst material on the porous medium enhances the efficiency of the operation of the photocatalyst material under visible light.
The aforesaid M may be one or more of Ti, Sn and Zn, and the aforesaid X is at least one of N, S, P, B, C, Cl, As, Se, Br, Sb, Te and I. These materials can be used to make a photocatalyst effective under visible light.
For the photocatalytic substance may include one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Re, Os, Ir, Pt, Mo and Nb, substituting at one or more titanium sites in Tixe2x80x94Oxe2x80x94X, doping between Tixe2x80x94Oxe2x80x94X crystal lattices, doping to crystalline grain boundary of Tixe2x80x94Oxe2x80x94X, or a combination of these methods. A simultaneous doping of both cationic and anionic speies enhances the catalytic activity of the photocatalyst.
By including such anionic species into an oxide, or by further doping cationic species, a new level is formed in a band gap of the oxide which becomes standard, resulting in a photocatalyst which absorbs visible light, as described in, for example, International Publication (PCT Gazette) Numbers WO 01/10552A1 and WO 01/10553A1 and, for titanium oxide, Japanese Patent Application No. 2000-19310.
The porous medium is preferably formed from ceramics such as alumina, silica, zirconia and titanium oxide; diatomaceous earth, zeolite, sepiolite and activated carbon, or from a composite or mixture of them. Such substances do not readily decompose when in contact with a photocatalytic material, and are therefore preferable for forming a porous medium for supporting the photocatalyst thereon.
Preferably, the porous medium comprises mesopores. It is further suitable that the porous medium has a honeycomb structure. Such porous medium can maintain a high catalytic activity of the photocatalyst material.